sql - Firebird 查询 - 包含下一行的值

Question

我有一个包含以下列的表格(非常简化):

CustomerID | ValidFrom | ValidTo
----------------------------------
1          | 20180101  | 20180330
2          | 20180601  | 20181003
1          | 20180212  | 20180313

我发现一个查询可以实现以下结果:

CustomerID | ValidFrom
-----------------------
1          | 20180101
1          | 20180212
1          | 20180314
1          | 20180331
2          | 20180601   
2          | 20181004

查询:

SELECT CustomerID, ValidFrom
FROM table
UNION
SELECT CustomerID, ValidTo + 1
FROM table

但是我怎样才能实现以下结果(几乎是上一行中包含的下一行的值)？

CustomerID | ValidFrom | ValidTo
---------------------------------
1          | 20180101  | 20180211
1          | 20180212  | 20180313
1          | 20180314  | 20180330
1          | 20180331  | NULL
2          | 20180601  | 20181003
2          | 20181004  | NULL

非常感谢!

Answer 1

您可以使用 SQL window functions 获取第二个查询的结果:

select
    validfrom
  , lag(validfrom,1) over (partition by customerId, order by validFrom)
  from (
      SELECT CustomerID, ValidFrom
      FROM table
      UNION
      SELECT CustomerID, ValidTo + 1
      FROM table
  ) foo